Security and/or value documents, such as passports, banknotes, and the like, require security against forgery, preferably in a way that allows such documents to be easily inspected for authenticity without the use of external aids.
As a result, it is desirable to incorporate into these products features whereby their authenticity may be checked.
Preferably, these security features should be fixed in an inseparable manner to the product that is to be authorized and/or they should have features that are difficult to reproduce. The invention relates to the manufacture and application of security features for security documents and/or value documents.
Numerous security features are known in the art, such as fluorescent fibres or threads/planchettes/inks, microprints, moiré-generating structures, holograms and the like.
Frequently the security features implemented in products requiring protection consist of special inks having special spectral characteristics, magnetic properties or temperature-dependent behaviour (thermochromic inks). Also used are barcodes which use the difference in reflectivity of the surface. Grid structures or even foil antennae that utilize the resonant frequency of an oscillator circuit are also used.
AU 488 652 discloses security documents where the security features can be inspected by transillumination. Arranged between plastic layers is an optically varying security element viewable through a transparent window in the overlying cover layer. The particular disadvantage is that the applied security features can only be viewed by the naked eye under transillumination, and that the material properties of the document are greatly altered.
The incorporation of a single transparent window on banknotes is also known in the art in Australian banknotes.
DE 43 34 847 A1 describes a value document formed with a window-like cut-out. The window-like opening is in the base material and subsequently covered by a transparent cover film. The cover film in this part of the window can incorporate additional security features. The process of manufacturing the cut-out opening in the base material of the security or value document, to form a window, and then covering this window with a cover film that partly incorporates additional security, is technologically expensive and results in an increase in the thickness of the value document. The requirement that certain parts of the window must remain transparent so that the security features can be viewed by transillumination with the naked eye limits the possibilities of providing security features in a concealed form. Like known window threads used on banknotes, such elements per se, only serve as a security element but have no further function. These known windows may themselves also incorporate security features.
Authenticity-checking of known security elements usually requires special equipment and methods, in particular: external testing devices such as magnifying glasses for checking microprinting, lenticular screens for checking hidden graphical elements, UV lamps to make fluorescent elements visible, or optical recognition of the respective security feature using a transillumination method.
DE 36 09 090 A1 describes a security or value document into which a security thread has been incorporated, which can be viewed visually without the use of technical aids. The security thread consists of a transparent material. Impressed into the upper surface is a screen of cylindrical lenses. Applied directly to the opposite side of the foil of the thread there is a printed image. The printed image is designed with consideration to the optical characteristics of the cylindrical lenses. By suitably designing the printed image accordingly, it is possible to incorporate information that only becomes visible from certain viewing angles.
A disadvantage here is that there is paper arranged directly beneath the thread resulting in the security feature only being viewable through reflection and not through transmission. The security lies solely in the threads which is a conventional security element.